The present invention relates to a reversible memory system which can thermally write information on a reversible memory medium, also thermally erase recorded information, and optically or magnetically read out information from the medium.
A memory device which can optically write, erase, or reproduce information in or from a reversible memory medium, i.e., a reversible memory disk, has been receiving a lot of attention recently due to high recording density and high random access speed. Such devices include magneto-optical memory media, which use the magneto-optical effect of a thin amorphous alloy film of a rare earth metal and a transition metal, and memory media which use phase transition between the amorphous and crystalline phases of metal.
However, mangeto-optical memory media are known to have the following problems:
(I) Reproduction of information is performed in accordance with the principle that the plane of polarization of linearly polarized light is rotated a maximum of several degrees. Since an optical system includes a polarizer and analyzer, it becomes complex in configuration and the S/N ratio of the reproduced signal is low.
(II) The thin amorphous alloy film of a rare earth metal and a transition metal as a memory medium can be easily oxidized in air. As a result, recorded information is lost, and the life of medium is short.
(III) Since a rare earth metal is used as a material for the recording medium, the cost is high as the costs of rare earth metal is high.
(IV) Since information is recorded by inversion of magnetic domains in the recording medium, the system must include a means for applying a magnetic field.
In a recording medium using phase transition between amorphous and crystalline phases, recording/erasure of information involves diffusion of atoms. Therefore, this type of medium is also susceptible to the following problems:
(I) Time required for recording or erasing 2-bit of information is long, thus maximum data transfer speed is limited.
(II) Thermal reversibility is gradually lost and some information may remain unerased after an erasure operation during heating/cooling of the medium being repeatedly performed.
Japanese Patent Disclosure No. 49-70547or No. 49-131041 comprise a memory medium which uses phase transition of a metal semiconductor. However, in this medium, changes in volume of recording area of a thin metal film upon phase transition are large, and the medium may peel from its base. The medium in Japanese Patent Disclosure No. 49-70547 must also have a means for constantly maintaining the overall medium at a high temperature in order to hold recorded information. Therefore, this medium has poor applicability. In the medium of Japanese Patent Disclosure No.49-131041, a region of mechanical separation must be formed between tracks for recording laser beam and tracks for erasure laser beam, so that a high memory density cannot be obtained.
Japanese Patent Disclosure No. 56-124113 proposes a memory structure having a first layer with a second layer formed thereon and kept in the martensitic phase at room temperature. This memory structure allows erasure of information by a shape memory alloy. However, this structure is known to have the following problems:
(I) In this structure, information is recorded such that part of a memory medium on a substrate deforms and floats off from the substrate. Therefore, a memory medium film having poor adhesion strength with respect to the substrate must be formed. Even if a memory medium film having good adhesion strength with respect to the substrate is used, the film easily peels off of the substrate.
(II) Since the memory medium consists of at least two different layers and the two layers are required to have proper adhesion strength respectively, the process for manufacturing such a memory disk is complex in procedure and the manufacturing cost of the disk is thus increased.